A fuel cell has high power generation efficiency, and is excellent in suppressing an environmental load. Accordingly, the fuel cell is a next-generation energy supply device expected to contribute to solution of environmental problems and energy problems, which have been current large subjects in countries which consume an enormous amount of energy.
Moreover, the fuel cell is classified in accordance with types of electrolytes. In particular, a polymer electrolyte fuel cell is compact and can obtain a high output. Accordingly, researches and developments have been progressed on application of the polymer electrolyte fuel cell as an energy supply source for small-scale stationary equipment, mobile unit and portable terminal.
An electrolyte membrane for use in such a polymer electrolyte fuel cell is a solid polymer material containing hydrophilic functional groups such as sulfonic acid groups and phosphoric acid groups in polymer chains, and has properties to selectively transmit cations or anions therethrough. From this fact, the electrolyte membrane is molded into a particulate shape, a fibrous shape or a filmy shape, and is utilized for a variety of uses such as electrodialysis, diffusion dialysis and a battery diaphragm.
Moreover, at present, the polymer electrolyte fuel cell has been actively improved as power generation means in which high comprehensive energy efficiency can be obtained. Main constituents of the polymer electrolyte fuel cell are both electrodes which are an anode and a cathode, separator plates which form gas flow passages therein, and a solid polymer electrolyte membrane that separates both of the electrodes from each other. Protons generated on a catalyst of the anode move in the solid polymer electrolyte membrane, reach a surface of a catalyst of the cathode, and react with oxygen. Hence, ion conduction resistance between both of the electrodes largely affects battery performance.
In order to form the above-descried polymer electrolyte fuel cell, it is necessary to couple the catalysts of both of the electrodes and such a solid polymer electrolyte to one another. Accordingly, in general, electrocatalyst layers are used, in each of which a solution of the solid polymer electrolyte and catalyst particles are mixed together, and both are coupled to each other by coating and drying. Then, the electrocatalyst layers and the solid polymer electrolyte membrane are pressed while being heated. Such a method is used.
Moreover, for the polymer electrolyte in charge of ion conduction, in general, a perfluorosulfonic acid polymer electrolyte is used. As specific commercial products, there are mentioned Nafion (registered trademark) made by DuPont Corporation, Flemion (registered trademark) made by Asahi Glass Co., Ltd., Aciplex (registered trademark) made by Asahi Kasei Corporation), and the like.
Such a perfluorosulfonic acid polymer electrolyte is composed of perfluorocarbon principal chains and side chains having the sulfonic acid groups. Moreover, it is considered that the polymer electrolyte is separated into micro phases which are: regions containing the sulfonic acid groups as a main component; and regions containing the perfluorocarbon principal chains as a main component, and that the phase of the sulfonic acid groups forms clusters. Sites where the perfluorocarbon principal chains are aggregated contribute to chemical stability of such a perfluorosulfonic acid electrolyte membrane. Sites where the sulfonic acid groups gather to form the clusters contribute to the ion conduction.